Method for calibration of milk meters in a milking system

ABSTRACT

A method for calibrating milk meters in a milking system includes a milking station having at least one milk meter that measures a value of a milking performance of a milking animal. The method includes the steps: determining a reference value which reflects the amount of milk received from a number of milking animals during a selected time period in a reference unit, retrieving all measured values during the selected time period for each milk meter that by itself contribute to the amount of milk received by the reference unit, comparing the reference value with the sum of all retrieved measured values and calculating a correction function for one of the milk meters, and using the calculated correction function to adjust the measured value from the milk meter.

TECHNICAL FIELD

The present invention relates to method for calibration of milk metersin a milking system, preferably an automatic milking system.

BACKGROUND TO THE INVENTION

In an automatic milking system, several milk meters are normally used tomeasure the amount of milk produced by the milking animals in a herd.Each milking animal may be milked using one or more of these milk metersdepending on the milking occasion. The milk meters used are calibratedon a routine basis to ensure that each of them measures the correctamount. This routine will entail unnecessary calibration of some milkmeters and at the same time there is a risk that other milk meters havebeen measuring an incorrect amount of milk during a period of time.

An obvious solution to reduce the number of incorrectly measuring milkmeters is to decrease the time between routine calibrations of all milkmeters. Another solution could be to regularly check and verify thefunction of each milk meter, to determine if a milk meter is in need ofa calibration, but this will decrease the through put in the automaticmilking system.

In an article with the title “A method for continuous automaticmonitoring of accuracy of milk recording equipment”, by G. Wendl, XZenger and H. Auernhammer, published in EAAP Publication No 65, 1992,pages 338 to 345, a method for identifying a malfunctioning or deviatingmilk meter is disclosed. The method only describes how to identify amalfunctioning milk meter by using previously recorded actual milkyields and comparing them with calculated expected milk yields. Themethod may also be adapted to automatic milking systems if the timelapsed since the last milking is entered into the calculation ofexpected yield.

When a malfunctioning milk meter has been identified, a manualre-calibration of the malfunctioning milk meter is performed. Thedescribed method is limited in use since it assumes that the systematicerror in measurement does not worsen at the same time on all milkmeters, that an error in measurement will drift in one direction(directed error), and that the other milking equipment has no defects.

SUMMARY OF THE INVENTION

The object of the invention is to provide a method for automaticallyrecalibrating at least one milk meter in a milking system.

An advantage with the present invention is that a verification of aproperly functioning milking system is performed automatically on aregular basis.

Another advantage is that it is possible to detect and correct asystematic error in measurement that does worsen at the same time on allmilk meters.

Still another advantage is that a manual calibration of a milk meter,which takes time and decreases the through-put in the milking system, isnot necessary. The calibration is instead performed by adding acorrection function to the output of the milk meter, which is in need ofcalibration, and is done fast and does not affect the through-put of themilking system.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in connection with the appendeddrawings, in which:

FIG. 1 shows a first embodiment of a milking system where the inventionmay be implemented.

FIG. 2 shows a second embodiment of a milking system where the inventionmay be implemented.

FIG. 3 shows a third embodiment of a milking system where the inventionmay be implemented.

FIG. 4 shows a fourth embodiment of a milking system where the inventionmay be implemented.

FIG. 5 shows a flow chart for measuring and storing milking performancevalues.

FIG. 6 shows a graph illustrating the method of calculating an expectedperformance value.

FIG. 7 shows an example of a lactation curve for a milking animal.

FIG. 8 shows a flow chart for calibration of a milk meter according tothe invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a first embodiment of a milking system comprising twomilking stations 10 a and 10 b. Each milking station is placed withinsome type of milking parlour and comprises four teat cups 11, which areattached to the milking animal during milking operations, and acollector 12, such as a claw. The milk from all teat cups passes thecollector 12 and is transported via line 13 to a milk meter 14 a and 14b, respectively. Several milk meters may be used for measuring themilking performance in each milking station, e.g. a milk meter for eachteat, but in this embodiment only one milk meter is used for eachmilking station 10 a, 10 b.

The milk meters 14 a and 14 b are separately and independently connectedto a control unit 15 via a communication line 16. Each milk metermeasures a value that corresponds to the milking performance of themilking animal in question, e.g. milk flow over time, the total weightor volume of the milk. The value corresponding to the milkingperformance of the animal is registered in the control unit 15,preferably in a memory or a database 17. The measured values are used inthe method according to the invention.

Furthermore, the control unit 15 comprises means to calculate anexpected milking performance value. The expected performance value isused to determine if a milk meter is in need of a calibration, asdescribed below. A display 18 is also connected to the control unit 15.

When the milk has passed each milk meter 14 a, 14 b the milk istransported to a common receiver 19.

FIG. 2 shows a second embodiment of a milking system comprising only onemilking station 20, preferably placed in an automatic milking systemusing a robot. The milking station 20 comprises four teat cups 11, whichare attached to the milking animal during milking operations. The teatcups are attached to a respective milk meter 21 a, 21 b, 21 c and 21 d,commonly denoted 21.

The milk meters are separately connected to a control unit 15 via acommunication line 16. The milk meters 21 measures the milkingperformance of the respective teat of the milking animal. The measuringof the milking performance is performed in a manner previously describedin connection with FIG. 1. The measured values are stored in a memory ora database 17 within the control unit 15 and are used in the methodaccording to the invention.

Also in this embodiment the control unit 15 comprises means to calculatean expected milking performance value. The expected performance value isused to determine if a milk meter is in need of a calibration, asdescribed below. A display 18 is also connected to the control unit 15.

FIG. 3 shows a third embodiment of the present invention comprising onemilking station 30 having four teat cups 11, which are attached to themilking animal during milking operations. The teat cups are attached toa common milk meter 31.

The milk meter 31 is connected to a control unit 15 via a communicationline 16. The milk meter 31 measures the milking performance of all teatsof the milking animal, in a manner previously described in connectionwith FIG. 1. The measured value is stored in a memory or a database 17within the control unit 15 and is used in the method according to theinvention.

Also in this embodiment the control unit 15 comprises means to calculatean expected milking performance value. The expected performance value isused to determine if a milk meter is in need of a calibration, asdescribed below. A display 18 is also connected to the control unit 15.

When the milk has passed the milk meter 31 the milk is transported to acommon receiver 19, which may have a sensor (not shown), such as a floatsensor, pressure sensors or a weight sensor, that measures the amount ofmilk in the receiver. The sensor, if present, is also connected to thecontrol unit 15 via a communication line 35.

When a milk truck 32 arrives to a milking system 36, as indicated by thedashed line, to transport the milk in the receiver 19 to a dairy plant,the tank of the truck 32 is connected to the receiver 19 via a milkmeter 33. The milk meter 33 is carried by the milk truck 32 and attachedto the inlet of the tank. The purpose of the milk meter 33 is to measurethe amount of milk collected from the milk system 36, i.e. the amount ofmilk in the receiver 19. This milk meter is normally calibrated often,and should therefore show an accurate value. The measured amount of themilk meter 33 may be fed back to the control unit 15 via communicationline 34, but it is also possible to manually feed this information intothe control unit 15.

FIG. 4 shows a fourth embodiment of the present invention comprising twomilking stations 10 a and 10 b. Each milking station is placed in sometype of milking parlour and each milking station comprises four teatcups 11, which are attached to the milking animal during milkingoperations. The teat cups of each milking station are connected to afirst milk meter 41 a and 41 b. The milk from each first milk meter isthereafter transported to an intermediate milk meter 42 a and 42 brespectively, via a collector 12, which preferably contains the wholevolume of the milk received from the udder. The milk meters 41 a, 41 b,42 a and 42 b are provided with means to measure a value correspondingto the milk performance and the first milk meters preferably measuresthe milk flow and the intermediate milk meters preferably measures theweight of the milk collected from the milking animal. The first milkmeters 41 a, 41 b are separately and independently connected to acontrol unit 15 via a communication line 43 and the intermediate milkmeters 42 a, 42 b are separately connected to the control unit 15 via acommunication line 44. Values from all milk meters corresponding to themilking performance of the animal in question are registered in thecontrol unit 15, preferably in a memory or a database 17. The measuredvalues are used in the method according to the invention.

The control unit 15 comprises means to calculate an expected milkingperformance value. The expected performance value is used to determineif a milk meter is in need of a calibration, as described below. Adisplay 18 is also connected to the control unit 15.

When the milk has been collected in the intermediate milk meter 42 a, 42b, and the amount of milk has been measured, the milk is transported toa common receiver 19, which in this embodiment has a sensor (not shown)that measures the total amount of milk from all present milking stations10 a, 10 b. The sensor is also connected to the control unit 15 via acommunication line 35.

When a milk truck 32 arrives at the milking system 40, indicated by thedashed line, to transport the milk in the receiver 19 to a dairy plant,the tank of the truck 32 is connected to the receiver 19 via a milkmeter 33. The milk meter 33 is carried by the milk truck 32 and attachedto the inlet of the tank. The purpose of the milk meter 33 is to measurethe amount of milk collected from the milk system 40, i.e. the amount ofmilk in the receiver 19. This milk meter is normally calibrated often,and should therefore show an accurate value. The measured amount of themilk meter 33 may be fed back to the control unit 15 via communicationline 34, but it is also possible to manually feed this information intothe control unit 15.

The milk meters in the above described types of milking systems 36 and40 may be monitored using the method according to the invention, whichwill be described in more detail below.

It is essential that the system comprises means to calculate an expectedmilking performance value at a given time for each animal in the systemaccording to FIGS. 1, 3 and 4, and for each teat in the system accordingto FIG. 2, if the system should be able to monitor and individuallyrecalibrate a milk meter that has an error in measurement. The expectedperformance value may be calculated in a number of ways, one of which isdisclosed in the article previously mentioned in the background to theinvention written by G. Wendl, X Zenger and H. Auernhammer.

FIG. 5 shows a flow chart describing the method for measuring a milkingperformance and collecting the information in a memory/database. Theflow starts, step 50, and the method determines if a new animal ispresent to be milked, step 51. If no new animal is present the flow isfed back to point 52. If a new animal is present the flow continuous tostep 53, where the identity of the animal is read, e.g. by means of atransponder, tags or similar means.

When the identity is determined the teat cups are attached to the teats,step 54, and the milking operation, step 55, commences. The milkingperformance is measured for each milk meter present in the milkingsystem, step 56. A milk meter may be measuring a parameter value thatcorresponds to the amount of milk from a part of the udder, or the totalamount of milk from the whole udder, depending on the type of automaticmilking system, see FIGS. 1 to 4. The values are stored in a memory ordatabase, step 57, together with information regarding the milk meterused, the point of time and the identity of the animal. This informationis needed to determine if a milk meter having an error in measurement.

The last step of the milking procedure is when the milk from all milkmeters is collected in the common receiver, step 58, and the flow isthereafter fed back to point 52. The amount of milk collected in thereceiver may also be measured and stored in the control unit 15, if ameans to measure the amount of milk in the receiver is present.

The calculation of the expected performance value may, as mentionedbefore, be done in several ways and include different components.

The first component that needs to be taken into account is at least onepreviously measured and stored milking performance value for the sameanimal as the calculation is made. A good approximation of an expectedperformance value is to take the previously stored milking performancevalue and use it as the expected performance value for next milkingoccasion, since the change in performance value between milkingoccasions normally is only minor provided the time between the milkingoccasions is approximately the same. However this simple approach hasthe drawback that an incorrect expected performance value may be used ifthe previous measured performance value was measured by a milk meterbeing in need of a calibration.

A better way of acquiring a reasonably good expected performance valueis to use several previously recorded milking performance values tocalculate a mean value over a selected time period, provided the timebetween the milking occasions is approximately the same.

The time between milking occasions in a voluntary milking system is as arule not the same. This is one of the advantages with that type ofsystem, since different animals have different needs when to be milked.Some animals prefer to be milked rather often compared to others. Thecalculation of the first component as described above is thus notapplicable. A different model is required. FIG. 6 shows a graph whereseveral previous measured milking performance values are used to createan expected milking performance curve. This curve is created for aselected animal from which the milking performance value for nextmilking occasion may be determined. This is done by arranging,preferably in a best fit manor, a straight line 61 through origin ofcoordinates and using the previously measured values 62. The expectedperformance value 63 is thereafter determined at the time t₁, which inthis example is approximately 16 liters due to the slope of the line 61.It is also possible to express the line 61 as an equation and thereaftercalculate the expected performance value there from

If many previously measured milking performance values are used tocalculate the expected milking performance value, the influence of anyincorrectly measured values will be reduced. Any previously measuredmilking performance value that deviate too much from the correspondingexpected performance value should be eliminated when calculating comingexpected performance values, as described below.

A second component that may be taken into account when calculating theexpected milking performance value is the shape of the lactation curveof each animal, which means that the expected value will depend on whereon the lactation curve the milking animal is. An example of a lactationcurve 70 is shown in FIG. 7. As can be seen the amount of milk producedby a milking animal varies over time. The lactation curve couldtherefore be used to further improve the expected performance value.

A third component for calculating the expected performance value is tomonitor the nutrition balance, e.g. water/feed intake of each milkinganimal, since this also will have impact on the amount of milk thatcould be produced.

A fourth component for calculating the expected performance value is tocompensate for the lactation cycle of each milking animal, since thismilking animal will produce a different amount of milk depending onwhich lactation curve the milking animal presently is in.

A fifth component for calculating the expected performance value is tocompensate if an animal is sick. The ability to produce milk may begreatly reduced during sickness.

An improved way to determine if a milk meter needs to be calibrated maybe performed by using the proposed method in the article mentioned inthe background to the invention. The discrepancy between actual andexpected milk yield is calculated from:d _(ikl) =m _(ikl) −M _(ik),

where

d_(ikl) equals deviation of expected milk yield from actual milk yieldof cow k on day i and meter l

m_(ikl) equals recorded actual milk yield of cow k on day i and meter l

M_(ik) equals expected milk yield of cow k on day i

The reliability of the monitoring method depends on the calculation of arealistic expected value. The expected yield and its standard deviationis calculated from

${M_{ik} = \frac{m_{{i - 7};k} + m_{{i - 6};k} + \ldots\mspace{11mu} + m_{{i - 1};k}}{u_{ik}}},{and}$${{SM}_{ik} = \sqrt{\frac{\sum( {m_{{i - x};k} - M_{ik}} )^{2}}{u_{ik}}}},$

where

M_(i-x;k) equals recorded actual milk yield of cow k on day i-x

SM_(ik) equals standard deviation of expected milk yield of cow k on dayi

u_(ik) equals number of available amounts of milk from cow k during theprevious 7 days

The expected yield and its standard deviation is calculated across theprevious 7 days in this example. To minimize the influence of anydeviating milk meter, only amounts of milk that have been recorded on atleast 3 different milk meters are used to determine if a specific milkmeter has to be calibrated.

Extreme values have to be identified and eliminated to calculate theexpected value. These extreme values may depend on the cow health,amount of nutrient received by the cow, environmental issues (e.g.introduction of a new member in the heard, etc.). The reason for theextreme values is not essential, but they must be eliminated tocalculate a realistic expected value. Some criteria are presented in thearticle, such as:

only milk yield from 30^(th) to the 300^(th) days in lactation are used.

an expected value is valid only if the coefficient of variation(SM_(ik)*100/M_(ik)) is below 20%.

if the standard deviation of the available amounts of milk is more than1.0, only the amounts in the range M_(ik)±2*SM_(ik) (i.e. 95.45% ofnormal distribution) are used to calculate a new expected value.

an expected yield is calculated only if at least 4 milk yield recordsare available across the previous 7 days, fulfilling the above mentionedconditions.

a deviation is only calculated if the actual milk yield is in the rangeM_(ik)±2*SM_(ik).

The calculation of average deviation for each milk meter and itsstandard deviation is calculated from:

${D_{il} = \frac{d_{{i - 30};k;l} + d_{{i - 29};k;l} + \ldots\mspace{11mu} + d_{{i - 1};k;l}}{x_{il}}},{and}$${{SD}_{il} = \sqrt{\frac{\sum( {d_{ikl} - D_{il}} )^{2}}{x_{il} - 1}}},$where

D_(il) equals running average of deviations of meter 1 at time i

SD_(il) equals standard deviation of deviations of meter 1 at time i

x_(il) equals number of available deviations of all cows during theprevious 30 days (interval 1-30 to i−1) on meter l

Additionally, it is assumed that the calculated deviations have a normaldistribution. Therefore the hypothesis H₀(D_(il)=0) can be testedagainst hypothesis H₁(D_(il)≠0). If the hypothesis H₀ is rejected over aperiod of seven running days, a milk meter error is signalled.

When a milk meter has been determined to be in need of calibration, thesystem may either alert the farmer by sending a message to the display18 or the system may automatically correct the malfunctioning milk meterby adding a correction function to the faulty, or deviating, milk meter.This is performed by the control unit 15.

To be able to correctly perform an automatic calibration, the systemneeds to have, in addition to the deviation values for each milk meter,access to a reference value which is used to control the calibrationprocess.

FIG. 8 shows a flow chart describing the calibration process, when amilk meter has been found to deviate, as described above.

The flow starts at step 80 and the process awaits a decision to proceedwith the calibration of a deviating milk meter in step 81. The flow isfed back in a loop to point 82 until a decision is made to proceed withcalibration of one or several milk meters. The process then proceeds tostep 83, where a reference value RV, which reflects the amount of milkreceived from a number of milking animals during a selected period in areference unit, is determined. The following examples will exemplify howa reference value is determined.

The process thereafter proceeds to step 84, where the measuredperformance values PV_(meas) are retrieved for each milk meter that byitself contribute to the amount of milk received by the reference unit.Each milk meter is directly or indirectly connected to the referenceunit. In step 85, the sum of the retrieved measured performance valuesis compared with the reference value and if a specific milk meter hasbeen found to be in need of calibration, that specific milk meter isadjusted so that the reference value is equal to the sum of the measuredperformance values. On the other hand if no milk meter has been found tobe in need of calibration, but the reference value still differs fromthe sum of the retrieved measured performance values, then all milkmeters may be adjusted so that the reference value is equal to the sumof the measured performance values, provided the reference value isconsidered to be an accurate value. If the reference value cannot beconsidered to be an accurate value the milk meters are not adjusted.

On the other hand if the reference value for instance has beendetermined by a newly calibrated milk meter (e.g. on a milk lorry) orthe receiver 19 has several independent sensors that together are usedto calculate the reference value, all milk meters may be adjusted if thereference value and the sum of the retrieved measured milkingperformance values differs more than the systems error margin. Inparticular, the correction function is selected to be equal to 1 unlessthe reference value (RV) deviates more than a predetermined amount fromthe sum of all retrieved measured milking performance values. Ingeneral, the predetermined amount is selected to be 5%. Also historicdata from the milk meters may be used to determine if all milk metersshould be adjusted.

The flow is fed back to point 82, awaiting a new decision to proceedwith another calibration procedure.

The method described in the article mentioned in background to theinvention, assumes that not all milk meters are faulty at the same time,but the method according to the invention actually takes care of that byusing an internal reference value (e.g. obtained from the sensor in thereceiver 19) or an external reference value, e.g. obtained from a milkmeter 33 arranged on a milk truck 32 that regularly transfer the milk inthe common receiver 19 to the truck 32. This milk meter is normallycalibrated at regular intervals and thus produces a very reliablereference value when the common receiver 19 is emptied and transferredto the truck 32.

During milking, the system may detect that some milk is “milk not forconsumption” e.g. contain bacteria etc., which means that the milk isdiscarded and thus not collected in the common receiver. When retrievingthe measured performance values that are directly or indirectlyconnected to the common receiver 19, the performance value thatcorresponds to the amount of the milk not for consumption has to beomitted. If not, the comparison between the reference value and sum ofthe measured performance values is misleading. The measuring of the milknot for consumption may still be used to control the milk meters usingthe method according to the article.

As mentioned above, the reference value does not have to be an externalvalue, but may advantageous be an internally generated value from adevice, such as a sensor or milk meter, having at least one milk meterconnected to it.

The method according to the invention will be described in the followingby a number of examples.

Example 1

This example will be exemplified using the milking system in FIG. 1.Each milk meter 14 a, 14 b measures the milking performance of oneanimal at the time. A corresponding expected performance value iscalculated using the proposed method from the article described above.The measured performance values are presented for both milk meters 14 a,14 b in table 1.

The common receiver 19 in FIG. 1 is provided with means (not shown) tomeasure the amount of milk present in the receiver. This means isnormally a sensor attached to the receiver, which typically measures theweight or volume of the milk,

The means to measure the amount of milk in the receiver is connected tothe control unit 15, and the signal from it is used as a reference valuewhen performing the calibration procedure.

TABLE 1 Milk meter 14a Milk meter 14b Milking Milking Milking occasionanimal PV_(meas) animal PV_(meas) 1 1 4.3 2 4.4 2 6 4.7 7 5.3 3 4 4.8 53.9 4 7 3.7 8 6.5 5 8 4.6 9 3.5 6 10 5.0 1 6.4 7 2 4.5 3 5.6 8 5 3.8 65.2 9 3 4.6 4 4.7 10 9 5.0 10 6.2 Σ PV_(meas,1) 45.0 Σ PV_(meas,2) 51.7

In this example the common receiver 19 did not contain any milk at thetime for milking occasion #1, and the reference value (RV) after milkingoccasion 10 corresponded to 92.3 liters of milk.

If a milk meter is found to be in need of calibration, e.g. milk meter14 b has been found to deviate, the calibration process is initiated byproceeding to step 83 in FIG. 8. The RV is established to be 92.3 litersand milk meter 14 a and 14 b are both directly connected to the commonreceiver 19. Therefore the sum of the measured performance values formilk meter 14 a and 14 b is compared with the RV and the faulty milkmeter is adjusted with a correction function C, e.g. a constant, anequation, etc. C is, in this example, calculated using the followingrelationship:RV=PV _(meas,1) +C*PV _(meas,2)

$C = {\frac{{RV} - {PV}_{{meas},1}}{{PV}_{{meas},2}} = {\frac{92.3 - 45}{51.7} = 0.915}}$

The sensor in the common receiver 19 may also in turn be calibrated whenthe receiver is emptied. The milk truck (not shown) that collects themilk from the milking system is provided with a milk meter attached atthe inlet of the tank, which measures the flow of the milk when thereceiver is emptied. The total amount of milk emptied from the receiver19 should correspond to the amount of milk measured by the sensor priorto the transfer of milk to the milk truck. If they differ, the controlunit may calculate a correction function for the sensor and therebycalibrate the means for measuring the amount in the receiver 19 at aregular basis, i.e. every time the receiver 19 is emptied.

A similar example could be made for the milking system illustrated inFIG. 2, with the exception that each milk meter only measures a quarterof the milk provided from the milking animal, since one milk meter isattached to each teat cup. The calibration procedure as described aboveis the same.

Example 2

This example will illustrate how all milk meters (including sensors inthe receiver 19) in a milking system may be calibrated when a milk truck32 empties the receiver 19.

In this example only one milking station is present, se FIG. 3, havingonly one milk meter 31 which communicates with the control unit 15, areceiver 19 provided with a sensor (not shown) which also communicateswith the control unit 15. When a milk truck 32 arrives to collect themilk in the receiver 19, a milk meter 33, attached to the inlet of themilk tank on the milk truck, is connected to the control unit 15 so thatthe system may receive a value corresponding to the amount of milktransported into the milk truck 32.

During a calibration procedure performed within the system, using themeasured milk volume in the receiver 19 as the internal reference valuefor calibrating the milk meter 31, the following correction function wascalculated:

$C = {\frac{RV}{{PV}_{meas}} = {\frac{92.3}{98.0} = 0.942}}$

This correction function was stored in the memory 17 of the control unit15. When the milk truck arrives and connects to the milking system, thefollowing values was accessible to the control unit 15.

Milk volume in receiver (measured by the sensor): 183.5 liters

Sum of all actually measured milking performance values of the milkmeter 31 since the last time the receiver 19 was emptied: 197.8 liters

The correction function, previously determined using an internalreference value, is stored in the control unit, will correct the sum ofall PV_(meas) to be 0.942*197.8=186.3, which is close enough to theamount of milk measured by the sensor in the receiver 19.

When the milk in the receiver 19 has been transferred to the milk truck,the control unit receives the actual milk volume from the milk meter 33,which in this example is 201.3 liters.

The control unit selects the measured milk volume from milk meter 33 asan external reference value and uses this information to recalibrate thesensor in the receiver by adding a receiver correction function, whichin this example is 201.3/183.5=1.097. The correction function for themilk meter 31 is also corrected by multiplying the previous correctionconstant with the receiver constant, e.g. 0.942*1.097=1.033.

The control unit in the milking system will now use the correctionfunctions when receiving measurement values from the milk meter 31 andthe sensor to calculate a calibrated value for them. In this way thereis no need to physically calibrate the milk meter or sensor, since anautomatic adjustment, implemented as a software related calibration, ismade in the control unit of the milking system.

Example 3

This last example is illustrated in connection with FIG. 4, which in itsbasic components is similar to the milking system in FIG. 1, with theexception that each teat cup 11 is connected to the collector 12 via afirst milk meter 41 a, 41 b. The first milk meter 41 a, 41 b measurespreferably the milk flow of each teat and the second milk meter 42 a, 42b measures the weight or volume of the total amount of milk.

In this system calibration checks may be performed in a number ofdifferent ways, using the measured milking performance values of a milkmeter to calibrate other milk meters attached to the milk meter that isused to establish the reference value. As an example the intermediatemilk meter 42 a may be used to determine if one of the first milk meters41 a needs to be calibrated, and the intermediate milk meter 42 b may beused to calibrate any of the first milk meter 41 b. The same may beapplied for receiver 19 and intermediate milk meters 42 a and 42 b, asdescribed in connection with example 3.

The examples that have been used to illustrate the method according tothe invention have been simplified to clearly point out certainfeatures.

1. A method for calibrating at least one milk meter in a milking systemcomprising at least one milking station having at least one milk meterthat measures at least one value of a parameter that corresponds tomilking performance of a milking animal, said milking station isaccessible to a herd of milking animals, said method comprising thesteps of: determining an internal or external reference value (RV) whichreflects an amount of milk received from a plurality of milking animalsduring a selected time period in a reference unit, retrieving allmeasured values during a selected time period for said at least one milkmeter that by itself contribute to the amount of milk received by saidreference unit, comparing said reference value (RV) with a sum of allretrieved measured values and calculating a correction function for atleast one of said milk meters which has been determined to be in acondition to cause errors in measurement, and therefore to be in need ofa calibration, and calibrating said at least one of said milk meters byusing said calculated correction function to adjust the measured valuefrom said at least one milk meter.
 2. The method according to claim 1,wherein said reference unit is selected to be a receiver that collectsthe milk in the system after milking of each milking animal, and saidstep of determining the reference value is performed by measuring theamount of the milk in the receiver, thus said reference value is aninternal reference value.
 3. The method according to claim 1, whereinsaid reference unit is selected to be an intermediate milk meter whichis directly connected to said at least one milk meter, and said step ofdetermining the reference value is performed by measuring a value of amilking performance parameter of said intermediate milk meter, thus saidreference value is an internal reference value, which may be compared tothe values measured by each milk meter.
 4. The method according to claim1, wherein said milking system is provided with an external unit beingprovided with an external milk meter to measure the amount of milktransferred from the milking system to said external unit, said step ofdetermining the reference value (RV) is performed by measuring theamount of milk transferred from a receiver to the external unit usingsaid external milk meter, thus said reference value is an externalreference value.
 5. The method according to claim 4, wherein the methodfurther comprises the additional steps of: determining the amount ofmilk in the receiver prior to transferring the milk to the externalunit, comparing said amount of milk in the receiver with the externalreference value, and calculating the correction function which is usedwhen determining the amount of milk in the receiver.
 6. The methodaccording to claim 4, wherein said method further comprises theadditional steps of re-calibrating the milk meters that by themselvescontribute to the amount of milk received by the receiver when thecorrection function has been calculated which is used when determiningthe amount of milk in the receiver.
 7. The method according to claim 1,wherein the correction function is selected to be equal to 1 unless thereference value (RV) deviate more than a predetermined amount from thesum of all retrieved measured milking performance values.
 8. The methodaccording to claim 7, wherein said predetermined amount is selected tobe 5%.
 9. The method according to claim 1, wherein the milking systemcomprises a control device connected to each milk meter, said internalor external reference value being accessible to said control unit, andsaid calculations of correction functions is performed in said controlunit.